Space and time distribution of subsurface H₂concentration in so-called “fairy circles”: Insight from a conceptual 2-D transport model

Abstract

<jats:p>Natural H2 emissions from the ground have now been measured in many places worldwide. These emissions can be localized on faults or be more diffuse in some sedimentary basins, usually of Proterozoic age. In such a case, emanation zones are often visible from aerial images or on high-resolution topographic maps since they correspond to slight depressions of circular to elliptic shape. Furthermore, the rounded depressions are covered with a scrubby vegetation which often contrasts with the surrounding vegetation. Although the emission structure displays a very regular shape, the distribution of H2 concentration in the first meter of soil in such a structure does show a clear pattern. For example, the maximum concentration is almost never measured in the center of the structure and the few time-resolved data show that the soil H2 concentration is variable with time. Here, the time and space evolution of H2 concentration is simulated using a 2-D advective-diffusive model of H2 transport in porous media. Several parameters have been tested as the depth and periodicity of the H2 point source (pulsed), bacterial H2 consumption and permeability heterogeneities of the soil. The radius of the structure is linked to the time spent by the H2 in the soil that depends on the soil permeability, the depth of the gas leakage point and the pressure of the bubble. To account for field observations, the case of a shaly, less permeable, heterogeneity in the center of the structures has been modeled. It resulted in an increase of the concentration toward the rim of the structure and a close to zero signal in its center. If the deep signal is periodic with a frequency smaller than a few hours, H2 concentration within the soil is almost constant; in other cases, the near surface concentration wave reflects the concentration periodicity of the source with a delay (in the range of 12 h for 30 m of soil) and so the near surface H2 concentration values will be highly dependent on the time at which the measurement is performed. H2 monitoring through a sensor network is thus mandatory to characterize the H2 dynamics in the soil of fairy circles.</jats:p>

Hydrogen Emanations in Intracratonic Areas: New Guide Lines for Early Exploration Basin Screening

“…We did not observe fairy circles in the bed of the rivers. The presence of clay in the river bed could be a causality since permeability strongly influences the H 2 migration pathway as modeled by [55] or even the intermittent presence of water. In this part of Brazil, the dry and rainy seasons are very contrasted.…”

The Chemistry of Hyperalkaline Springs in Serpentinizing Environments: 1. The Composition of Free Gases in New Caledonia Compared to Other Springs Worldwide

“…6). Such a very rapid (hourly) variation of the hydrogen concentration of gases emitted at the surface has been documented in the so-called "fairy circles" in Brazil [Myagkiy et al, 2020].…”

Can Weathering of Banded Iron Formations Generate Natural Hydrogen? Evidence from Australia, Brazil and South Africa

“…In other cases, the presence of H 2 has not been proven yet and we will hence just call them SCD. So far, assumptions have been made that these features represent the morphological expression in the landscape of a deeper on-going H 2 generation or leakage, even if these features and the related H 2 emissions can vary in space and time [23,24].…”

“…The relationship between the depth of the H2 generation and the size of the SCD is not direct, as it also depends on many parameters, such as the micro-organism activity (that consumes the H2) or the relative permeability of the soil (that influences the speed of the transport toward the surface) [24]. Nonetheless, knowing that South_Africa_1, South_Africa_2, South_Africa_3 and Australia_1 are located very close to iron mines, we consider the unimodal distribution as well as the range 0-200 m of equivalent diameter as From these graphs, it is tempting to consider the unimodal distribution as a characteristic pattern and the range 0-200 m as the characteristic equivalent diameter size of the SCD, while the wider distributions around larger average values might correspond to a set of initial and merged depressions explaining the high variability in features sizes.…”

“…The relationship between the depth of the H 2 generation and the size of the SCD is not direct, as it also depends on many parameters, such as the micro-organism activity (that consumes the H 2 ) or the relative permeability of the soil (that influences the speed of the transport toward the surface) [24]. Nonetheless, knowing that South_Africa_1, South_Africa_2, South_Africa_3 and Australia_1 are located very close to iron mines, we consider the unimodal distribution as well as the range 0-200 m of equivalent diameter as a characteristic pattern for SCD originated from a shallow depth H 2 and spread out generation zone, matching with water-BIF interactions.…”